In the art of sensing acceleration, a type of accelerometer described by a Peters U.S. Pat. No. 4,712,427 operates to produce four digital outputs. Two of these outputs are in the form of acceleration dependent frequency modulated signals. The other two outputs provide information concerning temperature and calibration coefficients respectively. To calculate the incremental change in velocity being experienced one may determine the changes in frequency between the first two outputs and provide adjustments in accordance with the second two outputs. Determining frequency changes is sometimes accomplished by means of a counter which counts the number of cycles of each output during a sample period. To obtain high accuracy, it is not only necessary to count the number of full cycles during a sample period but also any portions of a cycle which occur during that period. Having the number of full and partial cycles of the two acceleration dependent frequency modulated outputs, the average frequency during the sample period may then be numerically converted by a system processor to calculate the incremental velocity along the sensing axis of the accelerometer.
A full and partial cycle counting apparatus has been described in the Hulsing II, et al U.S. Pat. No. 4,786,861 which employs a first counter operable to provide an output count indicative of the number of full cycles in a sampling period defined by two successive strobe signals, a second counter which counts the integer number of high frequency clock cycles occurring during a full cycle of the unknown frequency and a third counter which counts the integer number of high frequency clock cycles during a time period starting with the end of the sample period and ending with the next rising edge of the signal being measured. Subtracting the ratio of the number of high frequency clock cycles during this latter time period to the number of high frequency counts during a full cycle from 1 is indicative of the portion of a full cycle that occurs before the end of the sample period. The ratio defined above indicates the portion of a full cycle that occurs after the start of the next sample period.
A problem encountered in the above described apparatus, arises from the fact that it is necessary to wait until after the end of a sample period to determine the number of high frequency clock periods existing in a portion of a cycle of the signal being measured. As a result, the information used to determine the incremental change in velocity along the sensing axis cannot be sent to the system processor for use until after the completion of the full cycle of the unknown frequency which was initiated prior to the end of the subsequent sample period.